Electrical heating apparatus

ABSTRACT

Electrical heating apparatus comprises first and second electric heater means. First means are provided in a control means for deriving from a source of electric power a first derived voltage consisting of first pulses all of the same polarity and for applying the first derived voltage pulse only to the first heater means. Second means are provided in the control means for deriving from the source of electric power a second derived voltage consisting of second pulses all being of opposite polarity to the first derived pulses and for applying the second derived voltage pulses to only the second heater unit. The first and second pulses occur alternately in time and do not overlap. The heater units each comprise at least one heating element comprising elongated resistive conductors adapted to dissipate heat and which have a maximum cross-sectional area equal to that of a circular conductor of a diameter not greater than 0.015 inches. The heating apparatus may comprise a forced circulation air heater with the conductors of the first and second heater units exposed to direct contact by the air stream. Alternatively, the apparatus can be used to heat a liquid, in which case the conductors may be embedded in refractory material and immersed in the liquid.

United States Patent [191 Smillie et al.

[4 1 Jan. 2, 1973 [54] ELECTRICAL HEATING APPARATUS [75] Inventors: George Horn Smillie; Frank Johnstone, both of Glasgow, Scotland Primary Examiner-A. Bartis Attorney-Cushman, Darby & Cushman [57] ABSTRACT [73] Asslgnee: EZZ SZZ gsg Llmlted Electrical heating apparatus comprises first and g second electric heater means. First means are pro- [22] Filed: Jan. 18, 1971 vided in a control means for deriving from a source of electric power a first derived voltage consisting of first [21] Appl' 106957 pulses all of the same. polarity and for applying the first derived voltage pulse only to the first heater [30] Foreign Application Priority Data means. Second means are provided in the control means for deriving from the source of electric power a Jan. 21, Great Britain Second derived voltage consisting f Second pulses all being of opposite polarity to the first derived pulses [52] US. Cl. ..2l9/364, 219/321, 219/375, and for applying the second derivedvoltage pulses to 7 219/486, 219/492, 219/501, 307/41 only the second heater unit. The first and second pul- [51] Int. Cl. ..H05b 1/02, H05b 3/02 ses occur alternately in time and not overlap The [58] Field of Search ..2l9/480-487, 501, heater units each comprise at least one heating 219/492, 493, 321, 364, 369, 370, 371, ent comprising elongated resistive conductors 374-376; 307/41 adapted to dissipate heat and which have a maximum 7 cross-sectional area equal to that of a circular conduc- References Clted tor of a diameter not greater than 0.015 inches. The heating apparatus may comprise a forced circulation UNITED STATES PATENTS air heater with the conductors of the first and second 638,236 l2/l899 Gold ..2l9/376 UX heater units exposed to direct contact by the air 1,217,229 2/1917 Smith ..2l9/364 stream. Alternatively, the apparatus can be used to 3,180,999 /1965 Kuy endall--- --.2l9/486X heat a liquid, in which case the conductors may be 3,335,319 1967 arner -.30 X embedded in refractory material and immersed in the 3,496,331 2/1970 Fleury et al ..2l9/486 X liquid 3,597,590 8/1971 Fleming ..2l9/50l 18 Claims, 10 Drawing Figures 120 E 3 Q E 12 l 5;;

uonm CUTOUT 78 e comoL mus PATENTEDJAH 2 I975 SHEET 1 [IF 3 MOTOR 5 M mm m wh a /r #0 MN 9 WW 6 ELECTRICAL HEATING APPARATUS This invention relates to electrical heating apparatus.

The invention provides in one aspect electrical heating apparatus comprising first and second heating elements, first means for deriving from a source of electric power a first derived voltage consisting of first pulses all of the same polarity, and for applying said first derived voltage only to the first heating element, second means for deriving from the source of electric power a second derived voltage consisting of second pulses and for applying the second derived voltage only to the second heating element, the second pulses all being of opposite polarity to the first pulses, said first and second pulses occuring alternately in time, the heating elements comprising elongated resistive heating conductors adapted to dissipate heat and which have a maximum cross-sectional area equal to that of a circular conductor of diameter not greater than .015 inches.

There may be in each element a multiplicity of said elongated conductors in parallel for each kilowatt of rated electrical power input.

The elements preferably comprise conductors having a cross-section area equal to that of a circular conductor of diameter not greater than 0.0076 inches.

The conductors may be supported on or by heat resistant insulating structure.

Thus, the conductors may be supported'on or by rods or strands of glass fiber or mineral fiber.

The conductors may terminate at a printed circuit terminal board.

In some embodiments, the apparatus may be apparatus for heating liquid and wherein the heating elements are embedded in refractory material or fibrous mineral material or particulate mineral material.

In such embodiments the elements may comprise conductors of diameter 0.006 inches or less, and/or non-circular conductors of cross-sectional area equal to that of such conductors.

The said first and second means may be such that each first pulse does not overlap in time the next succeeding second pulse, and such that each second pulse does not overlap in time the next succeeding first pulse.

The apparatus may be adapted to operate from a source of AC. electrical power, the said first and second means being adapted to derive and apply the first and second derived voltages to the first and second elements by connecting the first element to the source during part cycles of the source waveform of one polarity and by connecting the second element to the source during part cycles of the source waveform of opposite polarity, the first and second elements respectively being disconnected from the source during part cycles of opposite and one polarities respectively, the said first and second means being adapted to effect said connection and disconnection whilst the instantaneous power being drawn or about to be drawn from the source by the element being disconnected or connected is small compared to the mean power drawn by the said element when connected to the source.

Preferably the first means comprise a first unilaterally conductive means and the second means comprises a second unilaterally conductive means oppositely directed to the first unilaterally conductive means.

The said unilaterally conductive means may be switchable unilaterally conductive means.

There may be control means for controlling the said second means so that the second element is connected to the source during a given part cycle of said opposite polarity only if the first element was connected to the source during the immediately preceding part cycle of said one polarity.

The control means may be arranged to effect said control of the second means by switching the second unilaterally conductive means between operative and inoperative states.

The control means may comprise a capacitor arranged to be charged during said immediately preceding part cycle and to discharge during said given part cycle of opposite polarity thereby to permit the second means to connect the second element to the source during said given part cycle.

The capacitor may be arranged so that discharge thereof switches the second unilaterally conductive means into its operative state during said given part cycle.

The unilaterally conductive means may be adapted to be rendered operative in response to a control signal, there being a resistance-capacitance circuit for modifying the control signal so that in operation the elements are connected to and disconnected from the source when the said instantaneous power about to be drawn, or being drawn by the elements is small to the said mean power.

In another embodiment, the first and second unilaterally conductive means are connected in parallel to a bilaterally conductive electronic switch which is adapted to be rendered conductive in response to a control signal.

There may be a temperature sensing element adapted to sense the temperature of a medium heated by the apparatus, and to provide said control signal according to the sensed temperature.

There may he means for increasing the peak amplitude of the voltage pulses above the peak amplitude of the voltage of the source of electrical power.

The said elements may be disposed in a duct, a fan being provided to pass air downwardly through said duct, a thermal cut-out being disposed in said duct upstream of said elements whereby in operation if the fan fails, the cut-out senses hot air rising under free convection and disconnects the elements from the source of power. I I

' The invention will be described, merely by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 shows in diagrammatic form a heater according to the invention, intended for heating air;

FIG. 2 shows a heating element of the heater of FIG. 1;

FIG. 3 shows'an alternative form of part of the structure of FIG. 2;

FIG. 4 shows an alternative form of another part of the structure of FIG. 2;

FIG. 5 shows a circuit diagram of the heater;

FIG. 6 shows an alternative tom of the circuit of FIG. 5;

FIG. 7 shows a modification of part of the structure of FIG. 5;

FIG. 8 and 9 show further embodiments of the invention; and

FIG. shows in diagrammatic form a heater according to the present invention, intended to heat a liquid.

Referring to FIG. 1, an electrical heater suitable for heating air comprises during 10, wherein there are disposed two or more electrical heating elements 12,

each comprising a plurality of elongated conductors. A fan 14 is disposed above the heating elements, to pass air downwardly through the duct 10. A motor 16 is provided to drive the fan 14 to pass air at a rate such that the element conductors do not glow red hot when enerrality of conductors 22 each arranged in open-helical coiled form to dissipate heat when energized. The conductors are supported by rods 24 by means of heat resisting insulating structure comprising a number of discs 26 of insulating heat resistant material, which are maintained in spaced-apart relationship by collars 28 threaded on the rod. Each conductor 22 passes back and forth twice along the length of its supporting rod 24, so that its end are brought to respective terminals 30, 32, on a termination board 34. The rods 24 are conveniently supported, e.g., by means of screw-threaded ends and nuts 36 from the board 34. It will be noted from FIG. 2 that the conductors 22 are closely packed together, but, due to the discs 26, each individual conductor is maintained spaced from all other conductors, thus avoiding short circuiting, and permitting air to pass downwardly through the conductors to abstract heat therefrom. Each rod 24 and its associated supporting discs 26 may, if of sufficient size, support more than one conductor.

The conductors 22 are connected in parallel with each other. Thus, it will be seen from FIG. 2 that all terminals 30 are connected together, and all terminals 32 are connected together. The terminals 30 and 32 respectively being connected to input terminals 38, 40. The terminals 38, 40 are connected to the supply voltage via the control equipment as described hereafter.

The conductors 22 are of small cross-section, e.g. 0.015 inch diameter or less, compared to the cross-section (e.g. 0.024 to 0.128 inches diameter) of'heating' conductors normally employed in electrical heater elements of similar rated input. Thus, the conductors .22 may be wires of diameter 0.010 inch (33 Standard Wire Gauge) or less, and preferably wires of diameter 0.0076 inch (36 Standard Wire Gauge) or less. Indeed, provided the conductors 22 are suitably supported, e.g. by strands of glass fiber or other mineral fiber, it is possible to employ as the conductors wires of 0.004 inch diameter (42 Standard Wire Gauge) or less, perhaps even wires of 43, 44, 45, 46, 47, 48, 49 or 50 Standard Wire Gauge. Indeed, wires of less than 0.001 inch diameter are envisaged as the conductors. Of course any diameter below 0.015 inch, for example corresponding to one of the Standard Wire Gauges 29 to 41 may be employed. The wires are of a material conventionally used for electrical heating conductors, and their lengths are such that a multiplicity of conductors 22 in parallel are provided for each kilowatt of rated electrical powerinput. In this embodiment, there are at least four heating elements per kilowatt of rated electrical input, and preferably there are six, or eight or even more elements per kilowatt or rated electrical input.

FIG. 3 shows an alternative form of the terminal board 34. Instead of a series of terminals 30, 32 on the board, printed circuit connections 42, 44 are employed. Connections 46, 48 connect the termination board to the control equipment 20. The ends 50 of the conductors 22 are joined to the terminal strips, e.g. by brazing or soldering in a conventional manner.

FIG. 4 shows conductor and support arrangement which lends itself particularly well to use in conjunction with a printed circuit termination board such as shown in FIG. 3. A conductor 22 is coiled in open-helical form, and is of similar diameter of the conductors discussed hereinbefore. The number of conductors is similar to the number in the FIG. 2 embodiment. The coiling is supported loosely on a glass fiber rod 52, although tighter coiling may be employed if desired. Also, glass fiber strands can be substituted for the rods 52 when the conductors are very thin; An end 50 of the conductor is soldered or brazed to a printed circuit strip 42 or 44. The conductor 22 is wound along the length of the rod 52, and either joins at its other end a printed circuit strip on another termination board, or else returns similarly coiled along the length of another glass fiber rod or strand (not shown), parallel to the rod 52. The other end of the conductor is then soldered to the other printed circuit strip 42 or 44. Of course, other insulating materials in addition to glass fiber which have the necessary heat resistant properties may be employed.

If desired, the conductors 22, in any embodiment of the invention, may be non-circular, in which case the cross'sectional area of each conductor should be equal to that of a circular-section conductor having a diameter in the range already specified above.

Referring to FIG. 5, there is shown a circuitry embodied in the control unit 20.

The control unit has terminals 54,56 respectively to connect the heater to a source of electrical power. This source in this case is the mains electrical supply which provides an alternating voltage between a live line 55 and a neutral line 57. The thermal cut-out 18 is located in the live line immediately adjacent the terminal 54, so that if the fan 16 fails the electrical supply to the heating elements 12 is cut off. i

In FIG. 5 there are shown two identical heating elements 12, 12a each having a multiplicity of heating conductors 22, 22a. One heating element 12 is arranged in series with a thyristor 64 across the live and neutral lines 55 and 57. The heating element 12a is also in series with an oppositely-directed thyristor 66 across the live and neutral lines. It will be noted that both the thyristors 64 and 66 (which constitute switchable unilaterally conductive means) are directly connected to the live line 55.

The thyristor 64 is controlled by a control signal on a line 68 by means of temperature-sensitive switching responsive to the temperature of the air leaving the heater. Suitable switching is described and claimed in our copending British Pat. applications Nos. 34742/68 and 9443/70.

We have found that more satisfactory dissipation of heat may be obtainable when a unilaterally conductive means such as the thyristors 64, 66 are employed in combination with elements having conductors of small cross-section.

This we believe is because the volume of a conductor of given length and therefore its thermal capacity, varies with the square of its diameter, but the surface area of the conductor is directly proportional to its diameter. Thus, the ratio of the surface area to the volume of the conductor increases as the diameter thereof reduced. Consequently, for a given heat dissipation rate per unit surfacearea of the conductor in a given air flow, the surface temperature reduces with increase in the diameter. Conversely, for a given surface temperature and air flow, the heat dissipation rate per unit area is greater than with a conventional conductor.

Furthermore, due to the very small diameter of the conductors, we consider that for a given air flow the downstream surface of the conductor may dissipate heat more efficiently than does the downstream surface of a conventional conductor. This we believe is because the air flowing over the conductor has less tendency to break away from the downstream surface, due to the very small diameter of the conductor. With a conventional conductor wehave noted that because of flow break away, the downstream surface of the conductor may glow red hot. in any case, due to the very small diameter of the conductors used in this invention, even if flow break-away does occur, the close proximity of the upstream and downstream surfaces of the conductor may permit sufficient conduction of heat from the downstream to the upstream surface to avoid an excessive increase in the temperature of the downstream surface.

We consider that such thin wire conductors, if subjected to a conventional A.C. mains supply, or to a steady DC. voltage, would have a short life if required to produce a useful output. However, we have found that a useful output and reasonable conductor life may be obtained if the conductors are heated by derived voltages in the form of pulse trains, for example as derived in the FIG. 5 circuit.

This we believe is because the electrical power taken by the heater is proportional to the mean value of the square of the voltage applied to the conductors 22 thereof, whereas the heat transfer rate from the conductors of the air depends on the temperature of the conductors 22 which varies roughly linearly with the peak voltage applied to the conductors. Due to the very small diameter of the conductors 22, the temperature thereof may be increase very quickly when a voltage is applied thereto; more quickly than the heat generated can be dissipated to the surrounding air.

Thus, theheating element 12 responds more quickly when energized, and this enables its temperature and heat output to be maintained by supplying it with a derived voltage consisting of a train of voltage pulses, rather than with a continuous alternating voltage or steady DC. voltage which, to provide the same heat output, may cause the elements to burn out premature- The duration of each pulse is chosen with regard to the speed of response of the heating element, which decreases with increasing conductor diameter. With small diameter conductors as described herein the pulses can with advantage be derived from like parts of the cycles of the A.C. mains voltage, which have a period of 20 milliseconds.

Thus, the thyristor 64, being in series with the first element 12 connects it to the A.C. mains, and provides across this element a first derived voltage consisting of a train of first voltage pulses, each pulse being derived from only a given part of each cycle of the A.C. mains, during which part cycle the element is connected to the mains, and at the end of which it is disconnected from the mains. These part cycles all have one polarity, and approximate to half cycles, and thus so do the first voltage pulses.

The cathode of the thyristor 64 is connected via a control means comprising a diode 70, a capacitor 72 and another diode 74 to the neutral line 57. The control terminal of the thyristor 66 is connected to the junction between the diode and the capacitor 72.

In operation, assuming the appropriate voltage at its control terminal, the thyristor 66 conducts during a further like part of each cycle of the A.C. mains supply voltage, and connects the second element 12a to the mains supply during this part cycle and disconnects it therefrom at the end of the part cycle. These further like part cycles are of opposite polarity'to the part cycles of one polarity during which the thyristor 66 conducts. Thus, a second derived voltage consisting of a train of voltage pulses is applied to the second heater element 12a. The part cycles of opposite polarity and thus the second voltage pulses approximate to half cycles, and the first and second pulses occur alternately in time and approximately out of phase with each other. Thus since, as is preferable, the elements 12, 12a are of equal resistance, the heater as a whole offers a balanced load to the mains electricity supply.

By choosing the thyristors 64, 66 so that they have substantially identical characteristics, and so that they change between their conducting to non-conducting states as the A.C. voltage applied thereto passes through zero (or at least a voltage close to zero), it is arranged that each first pulse does not overlap in time the next succeeding second pulse, and that each second pulse does not overlap in time the next succeeding first pulse.

Furthermore, by making the thyristors zero-firing, or approximately so, as indicated above, each element 12, 12a is connected to or disconnected from the mains supply whilst the instantaneous power drawn from the source by the element is small (ideally zero) compared to the mean power drawn by the element whilst connected to the supply. This may reduce harmonic distortion in the mains supply, and the interference (e.g. radio interference) associated therewith.

In order to maintain the balanced nature of the load, the control circuit ensures that the thyristor 66 conducts during a given part cycle of opposite polarity to provide a second derived voltage across the element 12a only if the thyristor 64 conducted during the immediately preceding part cycle of said one polarity, to provide a first derived voltage across the element 12.

Thus, when the thyristor 64 conducts during a part cycle, the capacitor 72 is charged. During the next following part cycle of opposite polarity the capacitor 72 discharges via the control terminal of the thyristor 66, thus turning it on. If the control signal on the line 68 then is removed, the thyristor 64 does not donduct during the next part cycle of one polarity, and the capacitor 72 is not charged. Thus, no control signal is available for the thyristor 66, and it does not conduct either.

The control signal on the line 68 is obtained by the closure of a relay switch 68 controlled from a temperature-sensitive switching circuit 75 and a temperature sensor 76 as described in our aforementioned application Ser. No. 43742/68. The sensor 76 senses the temperature of the medium heated by the heater.

The control signal is derived from the A.C. mains supply and is modified by a resistance-capacitance timing circuit 77, 78, 79 which ensures that the thyristor 64 only starts to conduct as the mains supply voltage is small and increasing. If this were not done, the conduction of the thyristor would give rise to harmonic distortion in the mains supply since the element 12 at the instant of connection to the supply would draw a large power. The timing of course is effected by suitable choice of the values of the components 77, 78, 79 having regard to the type of the thyristor 64.

If desired, base load heating elements may be provided directly connected across the live and neutral lines 55, 57. Such base load elements are shown diagrammatically at 80, but it will be appreciated that if desired the base load heating elements may be of similar construction to those described with reference to FIGS. 1 to 4. Then they are used in circuitry as shown in FIG. 7 described hereafter.

FIG. 6 shows an alternative form of the logic circuit. Parts common to FIGS. 5 and 6 have the same reference numerals and will not be described again.

In FIG. 6 it will be noted that the positions of the element 12a (here shown diagrammatically), and the thyristor 64 are reversed. The logic circuit comprises, in parallel with the element 12, a series circuit comprising a diode 81, a ballast resistor 82, and a capacitor 83. The junction of the resistor 82 and the capacitor 83 is connected to the control terminal of the second thyristor 66 via a diode 84. The operation of this logic circuit is similar to that of the FIG. 5 logic circuit; when the thyristor 64 conducts during a part-cycle it charges the capacitor 83 and during the next part cycle of opposite polarity and capacitor 83 discharges via the control terminal of the thyristor 66, providing a control signal and rendering it operative enabling it to conduct. The diode 84 ensures that no control signal is present at the thyristor 66 when its cathode is positive to its anode (e.g. during the part cycles during which the thyristor 64 conducts).

As indicated previously, the surface temperature of the element 12 tends to follow the peak voltage applied thereto. Therefore, there may with advantage be provided a pulse-shaping network 86 which receives the mains voltage, and increases the peak voltage thereof so that the amplitude of the voltage pulses of the derived voltages are increased. Alternatively, a transformer may be employed.

In a yet further alternative, the heater may be arranged to operate from a direct-current supply, by

providing a D.C. to A.C. converter in place of the pulse-shaping network or transformer 86. The converter need not produce an A.C. supply having a sinusoidally varying waveform; in fact, the sharper the peaks of the waveform and the greater its amplitude the better. Of course, the pulse-shaping network or the transformer 86, or the DC. to A.C. converter may be provided with a circuit such as shown in FIG. 5.

If it is desired to obtain for short periods maximum heat output from the heater, regardless of economy and element life, then the thyristors 64 and 66 must be bypassed. To do this, there are provided ganged switches 65, 67 which when closed connect the elements 12, 12a directly across the live and neutral lines 55, 57.

The thyristors 64, 66 may be replaced by separate smaller thyristors in series with each conductor 22, 22a, or in series with groups thereof. The control terminals of the smaller thyristors replacing the thyristor 64 are then connected in parallel and so are the control terminals of the smaller thyristors replacing the thyristor 66.

FIG. 7 shows a modified base load heater for use in the FIG. 5 circuit in greater detail.

The base load heater has two identical elements 90, 92, each of one of the forms discussed with reference to FIGS. 2 to 4. In series with each element 90, 92 is a unilaterally conductive diode 94, 96. The diodes are both of the same type and thus each derives and applies to its respective element 90, 92 a derived voltage consisting of a train of pulses. The relative phasing of the pulses of each train is again approximately the pulses in each train thus occurring alternately.

It will be appreciated that in effect the base load heater is the same as that of FIGS. 5 or 6, except that the control circuitry and the temperature-sensitive circuitry are not provided.

Indeed, a simple manually switched heater can be provided by employing an arrangement as shown in FIG. 8. The components of the FIG. 7 heater (similarly referenced) are complemented by one or more manually-switchable pairs of elements and diodes 98,

100, 102, 104 of the same type as those of FIG. 7. High I or low output can be selected by the user by means of a manual switch 106.

FIG. 9 shows an improved version of the FIG. 8 heater. Parts already referred to carry the same reference numbers as before. In this heater the positions of the diodes 102, 104 are changed and they are connected in parallel to a bilaterally conductive electronic switch 108 (e.g. a triac or a quadrac) which replaces the switch 106 and which has a control terminal arranged to receive a control signal from a line 68 as in FIG. 5. A resistor and a capacitor 110, 112 function as a timing circuit similarly to the components 77, 78, 79.

It will be appreciated that, generally speaking, in all embodiments each of the heater elements is provided with a respective derived voltage having a cyclic voltage waveform consisting of alternately occurring peaks of relatively large amplitude and troughs of relatively small amplitude.

Although described with reference to an air heater, the present invention is applicable to other types of electrical heater, for example underfloor heaters or liquid heaters. Whilst in such applications the elements are not exposed to the air and the above-mentioned short-life problem may not arise, nevertheless the present invention may bring advantages in efficiency as a result of the thermal capacity of the heater elements being reduced due to the small cross-section of the conductors of such elements, enabling the heat output thereof to be maintained by derived voltage pulse trains.

In particular, FIG. 10 illustrates a heater for heating in water or other liquid 11; in which elements 12 are embedded in material 12" which is refractory material or fibrous mineral material or particulate mineral material such as magnesia, contained in a metal sheath 12a, the overall dimensions of the heater may be reduced. The ratio of surface area to volume thereof may also be increased perhaps tending to reduce the tendency of electric liquid heaters to derate with age. Control means, indicated at 20, may be of the type described hereinbefore.

Whilst the conductors of the elements in a liquid heater embodiment of the invention may be of any size below 0.015 inch diameter (or of equivalent cross-sectional area) we prefer elements of 0.0060 inch diameter (or equivalent cross-sectional area) or less.

We claim:

1. Electrical heating apparatus comprising:

first and second heating elements,

first means for deriving from a source of electric power a first derived voltage consisting of first pulses all of the same polarity, and for applying said first derived voltage only to the first heatint element,

second means for deriving from the source of electric power a second derived voltage consisting of second pulses and for applying the second derived voltage only to the second heating element,

the second pulses all being of opposite polarity to the first pulses,

said first and second pulses occurring alternately in time,

the heating elements comprising elongated resistive heating conductors adapted to dissipate heat and which have a maximum cross-sectional area equal to that of a circular conductor of diameter not greater than 0.015 inches.

2. Apparatus as claimed in claim 1 wherein there are in each element a multiplicity of said elongated con ductors in parallel for each kilowatt of rated electrical power input.

3. Apparatus as claimed in claim 1 wherein the elements comprise:

conductors having a cross-sectional area equal to that of a circular conductor of diameter not greater than 0.0076 inches.

4. Apparatus as claimed in claim 1 comprising heatresisting insulating structure for supporting the conductors.

5. Apparatus as claimed in claim 4 wherein the structure comprises rods of glass fiber.

6. Apparatus as claimed in claim 1 comprising a printed circuit terminal board whereat the conductors terminate.

7. Apparatus as claimed in claim I wherein the apparatus is apparatus for heating liquid, the heating elements being embedded in material chosen from the group consisting of refractory material, fibrous mineral material and particulate mineral material.

8. Apparatus as claimed in claim 7 wherein the elements comprise conductors of diameter 0.006 inches or less, and/or non-circular conductors of cross-sectional area equal to that of such conductors.

9. Apparatus as claimed in claim 1 wherein the apparatus is adapted to operate from a source of AC electrical power, the said first and second means comprising means to derive and apply the first and second derived voltages to the first andsecond elements by connecting the first element to the source during part cycles of the source waveform of one polarity and by connecting the second element to the source during part cycles of the source waveform of opposite polarity, the first and second elements respectively being disconnected from the source during part cycles of opposite and one polarities respectively, the said first and second means effecting said connection and disconnection whilst the instantaneous power being drawn or about to be drawn from the source by the element being disconnected or connected is small compared to the mean power drawn by the said element when connected to the source.

10. Apparatus as claimed in claim 9 wherein the said first means comprises a first unilaterally conductive means and the second means comprise a second unilaterally conductive means oppositely directed to the first unilaterally conductive means.

11. Apparatus as claimed in claim 10 wherein the said unilaterally conductive means are switchable unilaterally conductive means.

12. Apparatus as claimed in claim 11, wherein the unilaterally conductive means are adapted to be rendered operative in response to a control signal, there being a resistance-capacitance circuit for modifying the control signal so that in operation the elements are connected to and disconnected from the source when the said instantaneous power about to be drawn, or being drawn by the elements is small compared to the said mean power.

13. Apparatus as claimed in claim 12 wherein there is a temperature sensing element adapted to sense the temperature of a medium heated by the apparatus, and to provide said control signal according to the sensed temperature.

14. Apparatus as claimed in claim 10 comprising a bilaterally conductive electronic switch to which the first and second unilaterally conductive means are connected in parallel, the switch being arranged to conduct in response to a control signal.

15. Apparatus as claimed in claim 9 comprising control means for controlling the said second means so that the second element is connected to the source during a given part cycle of said opposite polarity only if the first element was connected to the source during the immediately preceding part cycle of said polarity.

16. Apparatus as claimed in claim 15 wherein the control means comprises a capacitor arranged to be charged during said immediately preceding part cycle and to discharge during said given part cycle of opposite polarity thereby to permit the second means to connect the second element to the source during said given part cycle.

17. Apparatus as claimed in claim 1 comprising means for increasing the peak amplitude of the voltage pulses above the peak amplitude of the voltage of the source of electrical power.

the duct, a fan being provided to pass air downsardly through the said duct, a thermal cut-out being disposed in said duct whereby in operation if the fan fails, the cut-out senses an increased temperature and discon- Apparatus as claimed in claim 1 comprising nects theelementsfrom thesource ofpower.

means defining a duct, the elements being disposed in 

2. Apparatus as claimed in claim 1 wherein there are in each element a multiplicity of said elongated conductors in parallel for each kilowatt of rated electrical power input.
 3. Apparatus as claimed in claim 1 wherein the elements comprise: conductors having a cross-sectional area equal to that of a circular conductor of diameter not greater than 0.0076 inches.
 4. Apparatus as claimed in claim 1 comprising heat-resisting insulating structure for supporting the conductors.
 5. Apparatus as claimed in claim 4 wherein the structure comprises rods of glass fiber.
 6. Apparatus as claimed in claim 1 comprising a printed circuit terminal board whereat the conductors terminate.
 7. Apparatus as claimed in claim 1 wherein the apparatus is apparatus for heating liquid, the heating elements being embedded in material chosen from the group consisting of refractory material, fibrous mineral material and particulate mineral material.
 8. Apparatus as claimed in claim 7 wherein the elements comprise conductors of diameter 0.006 inches or less, and/or non-circular conductors of cross-sectional area equal to that of such conductors.
 9. Apparatus as claimed in claim 1 wherein the apparatus is adapted to operate from a source of AC electrical power, the said first and second means comprising meaNs to derive and apply the first and second derived voltages to the first and second elements by connecting the first element to the source during part cycles of the source waveform of one polarity and by connecting the second element to the source during part cycles of the source waveform of opposite polarity, the first and second elements respectively being disconnected from the source during part cycles of opposite and one polarities respectively, the said first and second means effecting said connection and disconnection whilst the instantaneous power being drawn or about to be drawn from the source by the element being disconnected or connected is small compared to the mean power drawn by the said element when connected to the source.
 10. Apparatus as claimed in claim 9 wherein the said first means comprises a first unilaterally conductive means and the second means comprise a second unilaterally conductive means oppositely directed to the first unilaterally conductive means.
 11. Apparatus as claimed in claim 10 wherein the said unilaterally conductive means are switchable unilaterally conductive means.
 12. Apparatus as claimed in claim 11, wherein the unilaterally conductive means are adapted to be rendered operative in response to a control signal, there being a resistance-capacitance circuit for modifying the control signal so that in operation the elements are connected to and disconnected from the source when the said instantaneous power about to be drawn, or being drawn by the elements is small compared to the said mean power.
 13. Apparatus as claimed in claim 12 wherein there is a temperature sensing element adapted to sense the temperature of a medium heated by the apparatus, and to provide said control signal according to the sensed temperature.
 14. Apparatus as claimed in claim 10 comprising a bilaterally conductive electronic switch to which the first and second unilaterally conductive means are connected in parallel, the switch being arranged to conduct in response to a control signal.
 15. Apparatus as claimed in claim 9 comprising control means for controlling the said second means so that the second element is connected to the source during a given part cycle of said opposite polarity only if the first element was connected to the source during the immediately preceding part cycle of said polarity.
 16. Apparatus as claimed in claim 15 wherein the control means comprises a capacitor arranged to be charged during said immediately preceding part cycle and to discharge during said given part cycle of opposite polarity thereby to permit the second means to connect the second element to the source during said given part cycle.
 17. Apparatus as claimed in claim 1 comprising means for increasing the peak amplitude of the voltage pulses above the peak amplitude of the voltage of the source of electrical power.
 18. Apparatus as claimed in claim 1 comprising means defining a duct, the elements being disposed in the duct, a fan being provided to pass air downsardly through the said duct, a thermal cut-out being disposed in said duct whereby in operation if the fan fails, the cut-out senses an increased temperature and disconnects the elements from the source of power. 